James Webb Space Telescope (JWST) data have revealed hidden stars that astronomers say are unexpectedly increasing the mass of early galaxies.
JWST‘s ability to see deep into the cosmos gives astronomers insight into the early universe and challenges prior expectations of what they would find there. Now, an international team led by astronomers from the Netherlands’ Leiden University recently reported the discovery of new stars in a paper published in Nature Astronomy.
With the discovery of these many previously unknown small stars, the researchers say astronomers may need to reconsider the expected mass of these early galaxies, as JWST reveals unexpected levels of development shortly after the Big Bang.
James Webb Space Telescope Observations
For their new work, the astronomers focused on nine massive galaxies in the early universe that had completed their main star-formation period. They combined JWST data with earlier observations from the Very Large Telescope in Chile, operated by the European Southern Observatory.
Spectroscopy is a crucial element of JWST data, breaking light down into separate spectra that allow astronomers to study stars and even planetary atmospheres across extreme distances. However, the smaller, fainter stars revealed in this new research can be much harder to detect than the brighter, more powerful ones.
“If a galaxy were a city, the brightest stars would be the skyscrapers you can immediately see from far away,” said first author Chloe Cheng. “Our models show that behind the brightest stars there is a much larger population of small stars, like houses among the skyscrapers. This means that the galaxy as a whole is much more massive than previous estimates suggested.”
Stellar Birth
Working with data limited by the sensitivity of tools, researchers previously were unable to see as many smaller stellar objects. Researchers then assumed a similar ratio of small to large stars across the universe, which these new findings are now challenging.
Instead, they found ancient massive galaxies filled with many more small stars than previously expected, and far more than in our own Milky Way galaxy today, which is a smaller galaxy. The mass differences produced by this glut of tiny stars increased the expected mass of the galaxy under 1.5 billion years old by fourfold.
“Measurements like these were simply not possible until recently,” explained co-author Martje Slob. “We needed not only a telescope capable of collecting enough light, but also exceptionally high-quality spectra and new analysis techniques. Only then could we reliably distinguish the subtle features of small stars from the light of much brighter stars.”
JWST Discoveries
Launched almost five years ago, JWST has continually defied what astronomers expected to find in the ancient universe. It turns out that the universe shortly after the Big Bang was more massive and had undergone more development than previously considered possible. The new findings add to those ongoing discoveries.
“This result shows that much more mass than we thought is hidden in small stars,” said study lead Professor Mariska Kriek. “It has implications for many areas of astronomy. Small stars often have planets orbiting them, so if there were many more small stars in the early Universe than we thought, there may also have been more planets.”
To follow up on this exciting discovery, the team plans to look even further back into the universe’s past in future observations that will study some of the universe’s very earliest stars.
The paper, “Hidden Mass in Early Galaxies Revealed by Bottom-Heavy Initial Mass Functions,” appeared in Nature Astronomy on August 18, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org.